Electronic scanning radars of the prior art consist of directional antennas produced on the basis of radiating elements, or radiating cells, assembled within an array. Modification of the amplitude and of the phase of each of the radiating elements of the array makes it possible to steer the direction of the radar beam.
The frequencies of interest for aerial monitoring applications are the S band, used for the primary radar, and in particular the sub-band from 2.9 GHz to 3.3 GHz, as well as frequency bands of a few MHz or tens of MHz situated around the frequencies 1.03 GHz and 1.09 GHz, and used for IFF applications. Current radar equipment, whether they be ground-based radars or radars onboard a carrier such as for example a vehicle, a ship or an aeroplane, generally comprise two independent systems: a rotating directional antenna dedicated to IFF applications and an array of radiating cells for the S Band radar. The rotary antenna is positioned above or alongside the S band radar antenna. The two volumes are therefore added, and this may pose a problem when transporting or installing the antennas.
The invention seeks to solve the general problematic issue of the proliferation of systems by proposing a radiating cell operating simultaneously and without interference, in two distinct frequency bands, in particular the S Band and the frequency band dedicated to IFF applications. Such a cell makes it possible to produce a dual-band radiating array, thus reducing the footprint of the radar system as a whole, as well as the complexity of installation and the associated usage constraints. The invention proposes a radiating cell for which the feeds to the various frequency bands are independent, thereby making it possible to integrate the invention into existing radar devices in a transparent manner.
The use of dual-band or broadband radiating elements inside radiating arrays is a frequently encountered problem.
It is all the more complex as, when radiating elements are close together, strong coupling phenomena occur. These coupling phenomena are all the more marked when the ratio of the frequencies between the high band and the low band approaches an odd integer. Indeed, the radiating elements are dimensioned with respect to the wavelength at which they operate. An element dimensioned to radiate in the low frequency band will generally have a size close to λB/2, with λB the maximum wavelength of the low frequency band. On account of the ratio of the frequency bands, its size will also be N.λH/2, with N the ratio of the frequency bands and λH the maximum wavelength of the high frequency band in the dielectric. Therefore, when N approaches an odd integer, the device also radiates for the high frequency band, thus amplifying the coupling phenomena.
The use, within one and the same radiating cell, of elements that are specific to each of the operating bands and separated by a gap making it possible to minimize the problems of inter-element coupling, is not a solution to the problem when the radiating cell is implemented in a radiating array. Indeed, the size of the cell is constrained by the array mesh size, which generally equals λ/2, with λ the wavelength in air corresponding to the maximum frequency. Thus, when the ratio of frequencies between the high frequency band and the low frequency band increases, the radiating elements required by the low frequency band become incompatible with the size of this array spacing. By way of example, the array spacing of a mesh radiating in the S band at 3.3 GHz is about 5 cm. A patch adapted to the S band, when it is produced within the framework of a substrate having a relative dielectric constant of 3.55, has dimensions of the order of 25 mm×25 mm, compatible with the array spacing. A patch for IFF applications, on account of the frequency ratio of 3 between the two bands, will be 3 times as large (and 9 times bigger in area). Its size will then be 75 mm×75 mm. A device comprising a band S patch and a patch for IFF applications will not therefore be compatible with the radiating mesh size.
Thus, patent application US 2003/0164800 A1 presents a three-band device operating in the AMPS (800-850 MHz), GPS (1.4 GHz) and PCS (1.85-1.99 GHz) bands on the basis of a patch antenna and of two slots. The ratio of the operating frequencies not being odd multiples, the device does not exhibit any means of removing the interference related to the coupling between the radiating elements. Moreover, the use of a slot tuned to the low frequency band renders it incompatible with its integration into a radiating mesh dimensioned with respect to the high frequency.
Australian patent AU 2015101429 A4 presents a dual-band device operating in the Wifi bands at 2.4 GHz and 5 GHz. However, in this device, the ratio of the frequencies is not an odd multiple, it does not therefore exhibit any particular coupling problems. Nor does it exhibit independent feed to each of the frequency bands: the radiating elements associated with each of the frequency bands cannot then be driven independently.
A first known solution to the problem of producing a dual-band cell of reduced dimensions consists in using a single broadband radiating element. Once placed in an array, the result is then a single broadband array, covering all the bands of interest. However, the production of such a radiating element turns out to be complex when the band gap increases, and does not address the need for an independent feed to each of the frequency bands.
To address the problematic issue of the size of the array, a known solution consists in using, for the low frequency band, elements of folded monopole or dipole type, or slots folded in such a way that they can be accommodated in a reduced area. The simultaneous use of a patch for the high frequency band and of a slot for the low frequency band exhibits a practical interest, since the slot can be accommodated in the metallization of the patch, or in that of its ground plane. Diverse solutions of this type have been explored, but they come up against the fact that, under these conditions, radiating slots exhibit a very narrow passband, thereby limiting their interest.
The article “A Dual Band Quasi-Magneto-Electric Patch Antenna for X-band Phased Array”, S.E Valavan, Proceedings of the 44th European Microwave Conference 2014, has overridden this limitation by using the phenomena of coupling between the two elements. It proposes to disturb a radiating patch in the high frequency band with the aid of a slot accommodated inside the radiating area of the patch. The response of the device, resulting from the coupling between the two elements, exhibits operation in two distinct frequency bands whose central frequencies are a ratio of 1.5 apart, but having appreciable passbands (greater than 5%).
However, such a device exhibits two major defects:
the band ratio equals 1.5, which does not make it possible to address IFF and S Band radar applications, for which the frequency band ratio equals 3,
it does not address the need to have two separate antennas each linked to a distinct feed, since it proposes a coupled system having two resonance bands. The amplitudes and phases of the radiating elements associated with each frequency band cannot then be driven independently. Moreover, the integration of such a cell into existing equipment requires a separation between these two bands, in order to drive the signal in the high and low frequency band separately. This separation requires the production of an additional item of equipment at the interface between the radiating array and the radio equipment. It may turn out to be tricky, the quality of the resulting signals depending on the cleanness of the filtering implemented.